I said this in another tread, gonna say it again; Z-Moves Transformations are nothing but redundant. Why have two things that do the same ****?
While I agree, and think that it's dumb that Z-moves (in general, but especially transformations) and mega evolutions are coming off as different things, I'm going to explain how they could be different. One transforms on a whim, the other does a special attack THEN transforms. If I understand it correctly, Z-moves can only be used once per battle?
In addition, maybe they reveal that mega stones were just one type of Z-stone, the kind that lets you evolve, and that the rest of them are alternate types.
I think that Sun and Moon will be more like "Pokémon Z" than people are willing to believe.